By Alex Tresniowski
Updated July 05, 2010 12:00 PM

Thomas Mortimer IV spent June 13 touching up the paint on the pretty yellow house where his family lived, on the corner of Windsong Lane and Olde Village Drive in the leafy Boston suburb of Winchester. His young son Finn, 4 years old, held a brush in his tiny hand and helped out. “The boy would paint a little and ask his dad, ‘Is that okay?'” says their next-door neighbor Sam Bejakian. “His dad would say, ‘That’s good.’ It was all very touching.”

Just three days later, on June 16, paramedics broke into the Mortimer home and found Finn’s bloody body in a hallway and the body of his mother, Laura, 41, next to his. In the living room they found Laura’s mother, Ellen Stone, 64, dead and covered with a rug. And upstairs, in a crib soaked with blood, the body of Finn’s little sister Charlotte, who was just 2. Police soon tracked down the only missing member of the family, and the man they say brutally murdered them all: Thomas Mortimer.

Now those who knew the family must try to make sense of what one official called “horrific, disturbing and unspeakable” homicides, killings that were as barbaric-the victims were apparently attacked with sharp objects and bludgeoned-as they are mystifying. Mortimer, 43, a software salesman with no criminal record, left behind a typed note in which he admitted, “What I’ve done is extremely selfish and cowardly. I murdered my family,” according to Middlesex District Attorney Gerard Leone Jr. There was also evidence he tried to kill himself in the home (he has pleaded not guilty to four counts of first-degree murder). Interviews with several friends and neighbors revealed the couple had financial problems and constant arguments. But no one saw anything that even hinted at the horror to come. As a local pastor wondered during a June 17 memorial service, “How do we explain the inexplicable?”

Outwardly, at least, the Mortimers seemed a typical suburban family. Thomas, raised in comfortable Avon, Conn. (his father worked in the computer industry, his mother was a homemaker), and Laura, a bright and charming economist, were introduced in 2002 by Laura’s high school friend Miranda Simon. “They hit it off right away,” says Simon, 38. “They had this beautiful Martha’s Vineyard wedding, and Laura called me on every one of my birthdays to thank me.” Laura, her friends say, was living her dream: to have a husband and children. And Thomas, too, struck neighbors as devoted to his wife and kids. “He was always out in the yard playing with them, building little fortresses,” says one frequent visitor to their Winchester home. “He was very much involved with his kids.”

Their lives grew more complicated after Thomas lost his job at NetBrain Technologies, a computer-software company. Two years ago the Mortimers moved in with Laura’s mother in Winchester. (Ellen had long been separated but not divorced from Laura’s father, David Stone, a car dealer.) Says one neighbor: “It was a tense situation.” Thomas finally got a job with M&R Consultants, a technology firm, and started on June 1. “He delayed coming on for about two weeks until he could set up a good child-care situation for his kids,” says M&R President Anil Shah. “We all thought that sounded good, like he is a conscientious guy.”

But something was very wrong. On June 14, police say, Thomas and Laura had a long argument-“just another occasion of marital discord which has occurred between the two for quite some time now,” said prosecutor Leone. The next day Laura’s sister Debra called her cellphone and was surprised when Thomas answered. “It’s going to be a while before she can get back to you,” he told her. By then Thomas had already phoned Finn’s school to say his son wouldn’t be in and had called in for a sick day for himself.

The next day, June 16, Debra drove to the Winchester home, peered through a window and saw blood on the floor. She called 911 and paramedics found the bodies. Police put out an alert for Mortimer, and on June 17 Bernardston Police Chief James Palmeri pulled him over and arrested him. “He didn’t say anything,” says Palmeri. “He looked worn down.”

Mortimer, next due in court in August, is undergoing psychological evaluation. Some answers may come of that, but until then there is only a terrible question, How could such a beautiful family be wiped out, just like that? “We can really feel the emptiness there,” says Sam Bejakian of the yellow home that still bears signs of happier times: a plastic playhouse in the backyard, a child’s drawing taped to a window. “It would always be loud outside between the dogs and the kids and the parents, and we miss that,” he says. “Now it’s just very quiet.”